How to calculate pOH from pH? – (pH to pOH), Formulas, Equations
pOH determines the basicity of an aqueous solution just like pH measures its acidity. In this way, both pOH and pH are strongly interrelated like the opposite sides of the same coin.
If you are wondering how to find pOH from pH, then you are at the right place because, in this article, we will teach you how to calculate pOH from pH i.e. (pH to pOH) using a very simple but extremely functional chemical formula.
It is calculated by taking the negative logarithm of the concentration of OH– ions in an aqueous solution, as shown in equation (i).
pOH = -log10 [OH–]……….Equation (i)
As a base is primarily defined as a chemical substance that liberates OH– ions in water, thus pOH determines the basicity or alkalinity of an aqueous solution.
The greater the hydroxide ion concentration [OH–], the higher the basicity of an aqueous solution, thus lowering its pOH value.
In this way, pOH is inversely related to the basicity of an aqueous solution. Contrarily, it is directly related to acidity, which means that with increasing acidity, pOH increases and vice versa.
On a scale of 0-14, acidic solutions have a pOH above 7, while basic solutions have a pOH below 7. For instance, 0.1 M sodium hydroxide (NaOH) is a strongly basic solution having pOH 1 and pH 13.
What is pH?
pH stands for the power of hydrogen (H+) ions.
It is calculated by taking the negative logarithm of hydrogen ion concentration [H+] in an aqueous solution, as shown in equation (ii).
pH = -log10 [H+] ……….Equation (ii)
An acid is defined as a chemical substance that breaks down to release H+ ions in water. Thus, the greaterthe strength of an acid, the more H+ ions are released in its aqueous solution; consequently, [H+] increases.
However, the pH of the solution decreases as per equation (ii).
In this way, pH is inversely related to the acidic strength of an aqueous solution.
As per Bronsted-Lowry acid-base theory, acids are also defined as proton donors, while bases are proton acceptors.
So a strong base with low pOH readily accepts H+ ions from an aqueous solution; thus, the pH of the solution increases and vice versa.
On the pH scale, acidic solutions have a pH ranging from 0 to 6. pH 7 represents a purely neutral solution such as water, while a pH above 7 denotes the basicity of an aqueous solution.
What is the relationship between pOH and pH?
As discussed above, pOH is inversely related to pH. Greater the pOH of an aqueous solution, the lower its pH and vice versa.
Both pH and pOH are measured against a numerical scale having numbers from 0 to 14. Therefore, pOH is related to pH as per equation (iii), shown below.
pH + pOH = 14…………Equation (iii)
You may note that 14 = pKw represents water, a purely neutral solution. It is calculated by taking the negative logarithm of the water dissociation constant (Kw) at 25°C as shown in equations (iv) and (v).
pKw = -log10 Kw……………Equation (iv)
∴ Kw = 1.00 x 10-14 at 25°C
pKw = -log10 (1.00 x 10-14) = 14……….Equation (v)
How to find pOH from pH?
If we make pOH the subject of the formula, then equation (iii) given above transforms into equation (vi) shown below.
pOH = 14 – pH…………Equation (vi)
So, if the value of pH of an aqueous solution is known, we can find its pOH by substituting the given value into equation (vi).
Let’s understand this concept further by practicing with the solved examples given below.
Solved examples for calculating pOH from pH (pH to POH)
Example # 1: The pH of lime juice is 2.3. What is its pOH?
As the pH of lime juice is given in the question statement so we can easily find its pOH by applying equation (vi).
pOH = 14 – pH…………Equation (vi)
∴ pOH = 14 – 2.3 = 11.7
Result: The pOH of the given lime juice sample is 11.7.
Example # 2: A cleaning solution has a pH of 11.5. Calculate its pOH and determine whether this solution is acidic, basic, or neutral.
As the pH of the cleaning solution is given in the question statement so we can easily find its pOH by applying equation (vi).
pOH = 14 – pH…………Equation (vi)
∴ pOH = 14 – 11.5 = 2.5
Result: The pOH of the cleaning solution is 2.5. A low pOH value i.e., below 7, while a high pH value above 7 denotes that it is a basic solution.
Example # 3: For each solution listed below, calculate its pOH and determine whether this solution is acidic, basic, or neutral.
i) Solution A having pH = 9.25.
ii) Solution B having pH = 3.8.
As the pH of both solutions A and B is given in the question statement so we can easily find their pOH values by applying equation (vi).
pOH = 14 – pH…………Equation (vi)
i) pOH of solution A = 14 – 9.25 = 4.75
ii) pOH of solution B = 14 – 3.8 = 10.2
Result: As per the above calculation, the pOH of solution A is below 7, so it is basic, while that of solution B is above 7 thus, it is an acidic solution.
Example # 4: Both solutions C and D are basic. Determine by calculating their pOH values which one is more basic as compared to the other.
i) Solution C having pH = 10.5.
ii) Solution D having pH = 8.9.
As the pH of both solutions C and D is given in the question statement so we can easily find their pOH values by applying equation (vi).
pOH = 14 – pH…………Equation (vi)
i) pOH of solution C = 14 – 10.5 = 3.5
ii) pOH of solution D = 14- 8.9 = 5.1
Result: Both solutions C and D have low pOH values (below 7) thus, both are basic solutions. However, pOH (solution C) < pOH (solution D) thus, solution C is more basic than solution D.
Example # 5: What is the pOH of a 0.00340 M solution of nitric acid (HNO3)?
HNO3 is an acid that breaks down to release H+ and NO3– ions in water.
1 H+ ion is released per HNO3 molecule. Thus, [HNO3] = [H+].
As [HNO3] = [H+] = 0.00340 M is given in the question statement so we can substitute this value into equation (ii) to find the pH of the solution as follows:
pH = -log10 [H+] ……….Equation (ii)
pH = -log10 (0.00340) = 2.47
Now that the pH of the given nitric acid solution is determined, we can simply use equation (vi) to find the corresponding pOH.
pOH = 14 – pH…………Equation (vi)
pOH = 14-2.47 = 11.53
Result: The pOH of 0.00340 M nitric acid solution is 11.53.
A high pOH value reaffirms the strongly acidic nature of the nitric acid solution.
pOH denotes the negative logarithm of hydroxide (OH–) ion concentration. It determines the basicity of an aqueous solution.
The more basic an aqueous solution is; the more OH– ions are present in it; thus, the pOH of the solution decreases as per the following formula:
∴ pOH = -log10 [OH–]
What is pH?
pH stands for the power of hydrogen. It determines the acidity of an aqueous solution by taking the negative logarithm of hydrogen ion concentration [H+] present in the aqueous solution.
pH = -log10 [H+]
The greater the acidic strength of an aqueous solution, the lower its pH value and vice versa.
What is the relationship between pOH and pH?
pOH is inversely related to pH as per the following formula:
pH + pOH = 14
Greater the basic strength of an aqueous solution, the lower its pOH value; however, it has a high pH value.
Why does pH + pOH =14?
The formula pH + pOH = 14 is derived as follows:
(-log10 [H+]) + (-log10 [OH]–) = -log10 Kw
As –log10 [H+] = pH and -log10 [OH–] = pOH so the above equation becomes;
pH + pOH = -log10 Kw
Kw represents the water dissociation constant. At room temperature (25°C), 1 water (H2O) molecule dissociates to produce an equal number of H+ and OH– ions, as shown below.
Thus Kw = [H+] [OH–] = (1 x 10-7) (1 x 10-7) = 1 x 10-14.
pKw = -log10 Kw = -log10 (1 x 10-14) = 14
∴ pH + pOH = 14
On a numerical scale from 0-14, pH = pOH = 7 represents neutral water.
So acidic solutions have a pH below 7, while their pOH lies above 7 and vice versa for basic solutions.
For any solution X the sum of its pH and pOH values always stay the same i.e., 14.
How to calculate pOH from pH or convert pH to pOH?
pOH can be determined from pH by substituting the known value into the formula given below:
∴ pOH = 14 – pH
Summary
pOH stands for the power of hydroxide ions. It determines the basicity of an aqueous solution. It is calculated by taking the negative logarithm of hydroxide ion concentration [OH–].
pOH = -log10 [OH–].
pH denotes the power of hydrogen. It determines the acidity of an aqueous solution. It is calculated by taking the negative logarithm of hydrogen ion concentration [H+].
pH = -log10 [H+].
pH and pOH are inversely related to the acidity and basicity of an aqueous solution, respectively.
Strongly acidic solutions have low pH but high pOH values.
Strongly basic solutions have low pOH but a high pH value.
Both pOH and pH are measured on a numerical scale ranging from 0-14.
pOH is related to the pH of an aqueous solution by the formula: pH + pOH = 14.
If the pH of the given solution is known, we can find its pOH by substituting the known value into this equation: pOH = 14 – pH.
References
McCord, B. (n.d.). Chemistry 301. Chapter 6, Section 5: Acid-Base Equilibria. Retrieved from https://mccord.cm.utexas.edu/chembook/page-nonav.php?chnum=6§=5
Brightstorm. (n.d.). Tips for Calculating pH and pOH and More. Retrieved from https://www.brightstorm.com/science/chemistry/acids-and-bases/tips-for-calculating-ph-and-poh-and-more/
Enhanced Chemistry. (n.d.). Introduction to pH. Retrieved from https://ecampusontario.pressbooks.pub/enhancedchemistry/chapter/intro-ph/
Helmenstine, A. M. (2022, August 31). Quick Review of pOH Calculations. ThoughtCo. Retrieved from https://www.thoughtco.com/poh-calculations-quick-review-606090
Lumen Learning. (n.d.). pH and pOH. Retrieved from https://courses.lumenlearning.com/chemistryformajors/chapter/ph-and-poh/
Ammara Waheed is a highly qualified and experienced chemist, whose passion for Chemistry is evident in her writing. With a Bachelor of Science (Hons.) and Master of Philosophy (M. Phil) in Physical and Analytical Chemistry from Government College University (GCU) Lahore, Pakistan, with a hands-on laboratory experience in the Pakistan Council of Scientific and Industrial Research (PCSIR), Ammara has a solid educational foundation in her field. She comes from a distinguished research background and she documents her research endeavors for reputable journals such as Wiley and Elsevier. Her deep knowledge and expertise in the field of Chemistry make her a trusted and reliable authority in her profession. Let's connect - https://www.researchgate.net/profile/Ammara-Waheed
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